GnuPG and OpenPGP
=================
See RFC-4880 for a description of OpenPGP. These notes are older
than RFC-4880 and refer to the predecessor of the specs (RFC-2440).
Compatibility Notes
===================
GnuPG (>=1.0.3) is in compliance with RFC2440 despite these exceptions:
* With GnuPG >= 2.1.0 all support for version 3 keys has been
removed. Thus there is no more compatibility with PGP-2. Users
who need to be able to decrypt old PGP 2 messages should use
GnuPG 1.4.x along with the option --allow-weak-digest-algos.
* With GnuPG >= 2.1.0 all signatures (on messages and keys) are
created using version 4 signatures. Support for verifying
version 3 signature is still available.
* (9.2) states that IDEA SHOULD be implemented. This is not done
due to patent problems.
UPDATE: Since version 1.4.13 (or GnuPG 2.x with Libgcrypt 1.6)
IDEA support has been added to allow decryption of old
PGP-2 encrypted material.
All MAY features are implemented with this exception:
* multi-part armored messages are not supported.
MIME (rfc2015) should be used instead.
Most of the OPTIONAL stuff is implemented.
There are a couple of options which can be used to override some
RFC requirements. This is always mentioned with the description
of that options.
A special format of partial packet length exists for v3 packets
which can be considered to be in compliance with RFC1991; this
format is only created if a special option is active.
UPDATE: This support has been removed with version 1.3.6.
GnuPG uses a S2K mode of 101 for GNU extensions to the secret key
protection algorithms. This number is not defined in OpenPGP, but
given that this number is in a range which is used at many other
places in OpenPGP for private/experimental algorithm identifiers,
this should be not a too bad choice. The 3 bytes "GNU" are used to
identify this as a GNU extension - see the file DETAILS for a
definition of the used data formats.
Some Notes on OpenPGP / PGP Compatibility:
==========================================
* PGP 5.x does not accept V4 signatures for anything other than
key material. The GnuPG option --force-v3-sigs mimics this
behavior.
* PGP 5.x does not recognize the "five-octet" lengths in
new-format headers or in signature subpacket lengths.
* PGP 5.0 rejects an encrypted session key if the keylength
differs from the S2K symmetric algorithm. This is a bug in its
validation function.
* PGP 5.0 does not handle multiple one-pass signature headers and
trailers. Signing one will compress the one-pass signed literal
and prefix a V3 signature instead of doing a nested one-pass
signature.
* When exporting a private key, PGP 2.x generates the header
"BEGIN PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY
BLOCK". All previous versions ignore the implied data type, and
look directly at the packet data type.
* In a clear-signed signature, PGP 5.0 will figure out the correct
hash algorithm if there is no "Hash:" header, but it will reject
a mismatch between the header and the actual algorithm used. The
"standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
rejects the "Hash:" header and assumes MD5. There are a number
of enhanced variants of PGP 2.6.x that have been modified for
SHA-1 signatures.
* PGP 5.0 can read an RSA key in V4 format, but can only recognize
it with a V3 keyid, and can properly use only a V3 format RSA
key.
* Neither PGP 5.x nor PGP 6.0 recognize ElGamal Encrypt and Sign
keys. They only handle ElGamal Encrypt-only keys.
Parts of this document are taken from:
======================================
OpenPGP Message Format
draft-ietf-openpgp-formats-07.txt
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